Method of Anodizing Steel

ABSTRACT

A method of anodizing steel, wherein a steel object is connected to a positive terminal of a power supply, a counter electrode is connected to a negative terminal of the power supply, the steel object and counter electrode are placed into a solution of KOH or NaOH, and a voltage is applied across the terminals to anodize the steel object by forming a adherent blue-black or semi-adherent dichroic colored oxide coating thereon.

BACKGROUND OF THE INVENTION

The present invention relates to a method of anodizing iron or steel in particular non-stainless steel.

Bare steels rust when exposed to fresh water, salt water, or high humidity. The corrosion products on such steel after atmospheric exposure are flaky and non-adherent rust. The prior art methods of providing a barrier layer between the steel and the environment have proven to be unsatisfactory for many different reasons. It is therefore an object of the present application to provide a method of anodizing steel to form an adherent oxide coating on the steel.

BRIEF DESCRIPTION OF THE DRAWINGS

This objects and other objects and advantages of the present application, will appear more clearly from the following specification in conjunction with the accompanying drawings, in which:

FIG. 1 schematically indicates a test vessel for anodizing a steel object;

FIGS. 2 & 3 are graphs showing oxide film growth on steel objects;

FIG. 4 is a graph showing conditions for adherent blue-black and semi-adherent dichroic film formation on a steel object in 50% KOH;

FIG. 5 is a graph demonstrating the regions of adherent and semi-adherent oxide coatings grown on a steel object as a function of voltage and temperature in 25% NaOH; and

FIG. 6 is a graph showing the direct relationship of oxide film growth on a steel object as a function of both temperature and voltage in 50% KOH.

SUMMARY OF THE INVENTION

The method of anodizing steel pursuant to the present application includes the steps of connecting a steel object to a positive terminal of a power supply, connecting a counter electrode to a negative terminal of the power supply, placing the steel object and counter electrode into a 10% to saturated solution of KOH or NaOH, and applying a voltage across the terminals to anodize the steel object, wherein applying the voltage results in the formation of an adherent blue-black or a semi-adherent dichroic oxide coating on the steel object. “Dichroic” refers to a surface that reflects different colors when viewed at different angles.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring now to the drawings in detail, FIG. 1 schematically indicates how applicants' method of anodizing steel can be carried out. In the illustrated embodiment, a 50% solution of KOH is provided in an appropriate vessel 10. A steel object 12, which is preferably first cleaned and then rinsed with deionized water and then rinsed with methanol, is connected to the positive terminal 14 of a power supply 16, and a counter electrode 18 is connected to the negative terminal 20 of the power supply 16. The power supply is then turned on, and a voltage is supplied across the positive and negative terminals 14, 20, thereby anodizing the steel object 12. In particular, an adherent oxide coating or protective oxide film is formed on the steel object. This oxide coating is essentially a disordered or nanometer-size crystalline magnetite (Fe₃O₄). The solution is preferably rapidly stirred during the anodization process to obtain a uniform surface, and is also heated, as will be discussed subsequently.

Although the counter electrode 18 can also be made of steel, it could also be made of any other material that can conduct electricity and that does not corrode in KOH or NaOH, such as, by way of example only, platinum or nickel.

By way of example only, the electrodes formed by the steel object 12 and the counter electrode 18 can be spaced 9 cm apart for a two-electrode system using the voltages reported herein. It should furthermore be noted that a three-electrode system could also be used, and the required voltages would change accordingly.

The presently preferred concentrations for the electrolyte solutions are 50% KOH and 25% for NaOH. The 50% KOH solution can be prepared by adding deionized water to 500 g KOH to make one L of solution. Similarly, the 25% NaOH solution can be prepared by adding deionized water to 250 g of NaOH to make one L of solution. Tests resulting in the data of the graphs of FIGS. 2-6 were conducted using 50% KOH or 25% NaOH solutions.

Although it was indicated above that the solution could be heated, the temperature of the solution during anodization can be anywhere from room temperature to the boiling point of the solution.

FIG. 2 is a graph showing the dichroic colored oxide film grown on 1010 steel in a previously used 50% KOH solution, wherein adherent oxide films up to 120 nm in thickness were grown. Above 120 mm, the oxide films could be smeared with a cotton cloth, and were considered semi-adherent. The solution was at a temperature of 70° C. and a voltage of 2.0 volts was applied across the terminals. The graph plots the thickness of the oxide film grown as a function of time. In addition, the various colors of dichroic oxide film, as viewed from directly above, are shown along the right hand edge of the graph. For this plot, the tests were run in previously used KOH solution and chemical analysis of a used solution indicated a presence of 30 ppm dissolved Fe. Such presence of dissolved iron could potentially also be added to a fresh solution and appears to result in a faster growth of oxide.

FIG. 3 shows the effect of adding nitrates such as NaNO₃ to the 50% KOH solution. At the lower voltage of 2.5-2.7 volts, the growth of the oxide was accelerated. However, at the higher voltage of 5 volts, the nitrates retarded the oxide growth. Other additives, such as, by way of example only, NaNO₂, AlNO₃, Al and AlN, could also be used.

FIG. 4 illustrates the condition for adherent blue-black film formation (open crosses) and semi-adherent dichroic film formation (open circles) on 1010 steel in 50% KOH solution. The growth of adherent oxide of several microns thickness is possible anywhere within the region labeled “Adherent”, In particulars the adherent blue-black oxides can, for example, be grown at a temperature of 75° C. and a voltage of 1.8V, or a temperature of 110° and a voltage of 1.6V. The longer the time that the voltage was applied, the thicker was the resulting oxide film. The adherent oxide would not smear when wiped with a cotton cloth. Above 120° C., the adherent oxide changed colors and became a light brown adherent oxide, as shown in FIG. 4. Above 2.0V, the light brown oxide became semi-adherent and would smear when wiped with a cotton cloth.

FIG. 5 shows the dependence of the thickness of an oxide film growth on voltage and temperature for 1010 steel in a 25% NaOH solution over a period of five minutes. Again, the adherent blue-black film region is illustrated by open crosses, and semi-adherent dichroic colored oxide film region is shown by open circles. In particular, the blue-black oxide film growth occurred over a temperature range of from −65-80° C. and a voltage range of from 1.9-2.1 V.

FIG. 6 is a graph showing oxide film growth on interstitial free steel in a 50% KOH solution. Illustrated are the effect of both voltage and temperature upon the growth. Higher temperatures and/or higher voltages accelerated the growth.

For a uniform, thick, blue-black adherent anodic oxide, the preferred voltages to be applied across the terminals of the power supply range from 1.5 to 2.0V, and the temperatures range from 75-115° C., for a KOH solution, and 1.9 to 2.1V and 65-90° C. for a NaOH solution. The optimum conditions are shown in FIGS. 4 and 5. In addition. The voltage can be applied for from a few seconds to many hours, depending upon the desired thickness of the oxide coating that is to be formed. Finally, after the coating is obtained, either the blue-black or the dichroic colored coating, it could be thermally treated to convert the magnetite to hematite, thereby filling the oxide vacancies and making the oxide film a better barrier layer.

Potential applications for applicants' method of anodizing steel include corrosion protection, pre-weathering of weathering steels, conversion coating to improve the adherence of organic coatings, such as paints, internal protection of, for example, boiler tubes, and architectural colored highlights.

The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims. 

1. A method of anodizing steel, including the steps of: a) connecting a steel object to a positive terminal of a power supply; b) connecting a counter electrode to a negative terminal of the power supply; c) placing the steel object and counter electrode into a solution of KOH or NaOH; and d) applying a voltage across the terminals to anodize the steel object by forming an adherent blue-black or colored semi-adherent dichroic oxide coating on the steel object.
 2. A method according to claim 1, wherein said solution is stirred and/or heated during said step of applying a voltage.
 3. A method according to claim 2, wherein said oxide coating is essentially a disordered or nanometer-size crystalline magnetite (Fe₃O₄).
 4. A method according to claim 1, wherein said counter electrode is steel, platinum, nickel, or any other material that can conduct electricity and does not corrode in KOH or NaOH.
 5. A method according to claim 2, wherein said solution is at a temperature of from room temperature to the boiling point of the solution.
 6. A method according to claim 1, wherein said solution is KOH at a concentration of about 50%.
 7. A method according to claim 1, wherein said solution is NaOH at a concentration of about 25%.
 8. A method according to claim 1, wherein the voltage applied across the terminals is from 1.6 to 2.0 V for a KOH solution and 1.9 to 2.1V for a NaOH solution, in order to grow a black adherent oxide.
 9. A method according to claim 8, wherein said voltage is applied for from a few seconds to several hours, as a function a desired oxide coating thickness to be formed thereby.
 10. A method according to claim 8, wherein the temperature of said solution is from 75-140° C. for KOH and from 40-80° C. for NaOH.
 11. A method according to claim 1, wherein the voltage applied across the terminals is greater than 2.0 V for a KOH solution, and greater than 2.2V for a NaOH solution, and the temperature and the time of application are such as to obtain a specified color of dichroic oxide film. 